1. Field of the Invention
This invention relates to a distortion surveillance apparatus for detecting an abnormal increase of signal distortion occurring in a trunk amplifier or the like used in the transmission system of television signals such as a CATV system.
2. Description of the Prior Art
Recently, CATV systems have tended to have a large number of channels. Accordingly, a particularly low distortion characteristic is required for the signal transmission system, especially for the trunk amplifier. On the other hand, since CATV systems have tended to also have a large number of subscribers and a large service area, the number of trunk amplifiers have increased. In such circumstances, the main factor which lowers the reliability of the CATV transmission system is mostly related to the trunk amplifiers, which have active elements.
As one of the problems relating to a trunk amplifier, an increase in signal distortion due to excessive signal levels in the amplifier is known. If the occurrence of such an increase in signal distortion can be detected earlier and the defective position can be identified, the time required to restore the system can be reduced. Such a function can be realized by a remote-monitoring distortion surveillance apparatus.
Conventionally, to discover such problems an increase in the signal distortion, a pilot signal of a specific frequency is transmitted, and the level of the pilot signal is detected at the input or output of the trunk amplifier, so as to thereby determine whether or not the transmitted television signal level is proper. Based on this determination, an excessive signal level is detected, thereby indirectly discovering the problem causing the increase in the signal distortion. Such a problem detecting unit is installed in the trunk amplifier, and has a one-way or two-way communication function. The status of each trunk amplifier is generally surveyed by exchanging data with the surveillance terminal installed at the head end or at another location (see, for example, Japanese Unexamined Patent Publication No. 50-33723).
In such method, however, it may not be always possible to detect the distortion which occurs when the input of the line amplifier becomes excessive at a certain frequency due to a disturbance of the in-band frequency characteristics. This is because the detecting unit is responsive only to the frequency of the pilot signal. Or, if the line amplifier has a push-pull construction, the trouble cannot be detected when the distortion is increased due to a problem in the line amplifier itself, such as an improper balance of this unit, while the pilot signal level is normal.